1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine configured to form an image by an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
In the image forming apparatus employing an electrophotographic method or an electrostatic recording method, a toner image formed on a surface of a photosensitive drum is transferred onto a sheet as a recording medium so that the image is recorded on the sheet. In general, as for the transfer of the toner image, toner on the surface of the photosensitive drum is electrostatically transferred onto the sheet by an application of a bias to a transfer roller while the sheet is conveyed by being nipped between the photosensitive drum and the transfer roller.
A method of applying a bias to the transfer roller includes a constant voltage control method and a constant current control method. The constant voltage control method is a method of applying a transfer bias so that a voltage to be applied is maintained at a constant value. The constant voltage control method has been conventionally widely used. However, according to the constant voltage control method, in a case of small-sized sheets, a current flows intensively through a region in which the transfer roller is in direct contact with the photosensitive drum, and hence a current cannot be sufficiently supplied to the small sheets so that a transfer failure may occur.
In contrast, according to the constant current control method, the transfer bias is applied so that a constant current flows through a sheet irrespective of a size of the sheet, and the current is compensated when the size of the sheet is small. Under the circumstance, in recent years, the constant current control method has been more widely used as the method of applying a transfer bias. In the constant current control method, it is necessary to set a voltage for passing a constant current through the transfer roller. Thus, conventionally, a constant current, which is supposed to flow at the time of transfer, is flowed through the transfer roller at a time of non-image formation, specifically, prior to the start of an image forming operation, and a voltage applied at that time is maintained and applied at a time of image formation (Japanese Patent Application Laid-Open No. H07-146619).
However, an electric resistance of the transfer roller varies depending on an environment (temperature and humidity) in which the image forming apparatus is installed and a long-term use. In extreme cases, a value of the electric resistance may fluctuate by an order of magnitude or more. Thus, when the resistance of the transfer roller is higher than usual, a voltage to be applied to flow a target current also increases. When the voltage to be applied exceeds a predetermined value, a separation electric-discharge phenomenon occurs near a nip between the photosensitive drum and the transfer roller. The separation electric-discharge occurs when a sum of a strength E1 of an electric field between the photosensitive drum and a sheet and a strength E2 of an electric field between the transfer roller and the sheet exceeds a strength Em of a discharge start electric field. In particular, the separation electric-discharge is liable to occur when a high voltage is applied to the transfer roller. When the separation electric-discharge occurs, toner transferred on the sheet is scattered so that the scattered toner may pose a problem of griming an inside of a main body of the image forming apparatus.
However, when the voltage to be applied is kept as low as possible in order to prevent occurrence of the separation electric-discharge, the current flowing through the sheet at the time of transfer becomes smaller, which may cause transfer failure of the toner.